The present invention is directed to an integrated optical device, and more particularly to an improved grating assisted, directional coupler.
Many different types of integrated optical devices use waveguide couplers to couple light from one waveguide to another. One particular type of waveguide coupler is a grating-assisted coupler, which is often used as a transmission filter. In a grating-assisted coupler, the two waveguides have different effective refractive indices, and a grating close to one or both of the waveguides is used to phase match the coupling of light from one waveguide to the other.
The bandwidth of a grating-assisted coupler is typically inversely proportional to its length, and so a longer grating-assisted coupler generally results in a smaller transmission bandwidth. Smaller bandwidths are desirable in certain applications, for example when used as a frequency selective element with a semiconductor laser. On the other hand, the grating introduces scattering losses and longer couplers may suffer from higher loss than shorter couplers. Therefore, a compromise is often made between transmission losses and bandwidth. It is important, therefore, that the coupler operate optimally so that the best combination of bandwidth and transmission loss may be obtained.
Generally, the present invention relates to an approach to improving the operation of a grating-assisted coupler. In particular, the mode of the first waveguide of the coupler may include some electric field in the second waveguide, with the effect that when light passes from an input waveguide to the first waveguide, some light is launched, or injected, directly into the mode of the second waveguide. This injected light may or may not be in phase with the light subsequently coupled into the side of the second waveguide from the first waveguide via grating assistance. The grating structure is formed to ensure a desired phase relationship between the injected light and the grating coupled light: under certain conditions of relative phase, the transmission through the coupler may be increased and the bandwidth may be reduced.
In one particular embodiment, the invention is direction to a grating-assisted coupler device that includes a first input/output (i/o) waveguide and a coupler unit. The coupler unit has a first coupler waveguide coupled at a first end to the first i/o waveguide, and a second coupler waveguide disposed proximate the first coupler waveguide. Light propagating from the first i/o waveguide into the coupler unit launches a first portion of light into the first coupler waveguide and a second portion of light into the second coupler waveguide. A grating structure is disposed proximate at least one of the first and second coupler waveguides to couple light from the first coupler waveguide to the second coupler waveguide. The grating structure includes a periodic structure having a starting end proximate the starting end of the second coupler waveguide and positioned so that the light coupled by the grating structure into the second coupler waveguide from the first coupler waveguide is substantially in phase with the second portion of light launched into the second coupler waveguide.
Another embodiment of the invention is directed to a laser having a gain region including a gain waveguide, and a coupler region including a first coupler waveguide coupled at a first end to the gain waveguide. The coupler region also includes a second coupler waveguide proximate the first coupler waveguide. Light propagating from the gain waveguide into the coupler region launches a first portion of light into the first coupler waveguide and a second portion of light into the second coupler waveguide. A grating structure is disposed proximate at least one of the first and second coupler waveguides to couple light from the first coupler waveguide to the second coupler waveguide. The grating structure includes a periodic structure having a starting end proximate the starting end of the second coupler waveguide and positioned so that the light coupled by the grating structure into the second coupler waveguide from the first coupler waveguide is substantially in phase with the second portion of light launched into the second coupler waveguide. A reflector region has a reflector waveguide coupled to receive light from the second coupler waveguide.
Another embodiment of the invention is directed to a an optical communications system that has an optical transmitter having at least one laser, a fiber optic link coupled to receive light signals output from the optical transmitter, and an optical receiver unit coupled to detect the light signals from the fiber optic link. The at least one laser includes a gain region including a gain waveguide and a coupler region having a first coupler waveguide coupled at a first end to the gain waveguide. The coupler region also has a second coupler waveguide disposed proximate the first coupler waveguide. Light propagating from the gain waveguide into the coupler unit launches a first portion of light into the first coupler waveguide and a second portion of light into the second coupler waveguide. A grating structure is disposed proximate at least one of the first and second coupler waveguides to couple light from the first coupler waveguide to the second coupler waveguide. The grating structure includes a periodic structure having a starting end proximate the starting end of the second coupler waveguide and positioned so that the light coupled by the grating structure into the second coupler waveguide from the first coupler waveguide is substantially in phase with the second portion of light launched into the second coupler waveguide. The laser also includes a reflector region having a reflector waveguide coupled to receive light from the second coupler waveguide.
Another embodiment of the invention is directed to a method of forming a coupler that includes forming a first coupler waveguide in a coupler region and forming a second coupler waveguide in the coupler region over the first coupler waveguide. A grating structure is formed proximate one of the first and second coupler waveguides. The grating structure includes a periodic structure having a starting end positioned relative to a starting end of the second coupler waveguide so that light coupled by the grating structure from the first coupler waveguide into the second coupler waveguide is substantially in phase with light injected into the starting end of the second coupler waveguide when passing light into a starting end of the first coupler waveguide.
The above summary of the present invention is not intended to describe each illustrated embodiment or every Implementation of the present invention. The following figures and detailed description more particularly exemplify these embodiments.